1. Field of the Disclosure
The present disclosure relates to a method for preparing graphene using a spontaneous process, and particularly, to a method for mass-producing high-quality graphene composed of a single layer or several layers by utilizing lithium intercalation of a graphite electrode occurring during the process of charging a lithium ion battery and a lithium ion capacitor in the preparation of graphene to form a graphite intercalation compound, and performing exfoliation through a reaction with water (or alcohol).
2. Background of the Disclosure
Graphite has a layered structure in which graphene having a plate structure formed by connecting carbon atoms in a hexagonal ring and having a monoatomic thickness, is stacked, and a distance between the layers is 3.35□. Graphene has a structure in which a single-layered carbon nanotube is stretched out, and thus is a material which has high conductivity corresponding to that of carbon nanotubes, excellent mechanical properties, and a large surface area due to a high aspect ratio. Since layers of graphite are weakly bonded to each other by Van der Waals forces, many studies on the preparation of expanded graphite in which these layers are partially exfoliated, or graphene in which these layers are completely exfoliated have been conducted. Graphene has many advantages such as excellent charge mobility, low surface resistance, excellent mechanical properties, and thermal and chemical stability compared to other materials. Recently, many applied studies in which these excellent advantages of graphene are utilized have been reported. In particular, graphene is a very excellent conductor having 100 times or more charge mobility than that of silicon and is evaluated to enable electric current to flow in an amount 100 times greater than that of copper, and thus has technical and industrial application values in various fields such as an electrode material for a display and a solar cell, a channel material for transferring charges of a next-generation semiconductor device, and an additive of a conductive polymer film by utilizing these characteristics.
As a result of peeling off a graphene layer from graphite and investigating the properties thereof, it has been recently found that graphene has very useful properties which are different from those of existing materials. The most noteworthy property is that when electrons move in graphene, the electrons flow as if the mass of the electrons is zero. This means that the electrons flow at a speed at which light travels in vacuum, that is, at the speed of light. Graphene also exhibits an abnormal half-integer quantum Hall effect with respect to electrons and holes. Further, the electron mobility of graphene is as high as about 20,000 to about 50,000 cm2/Vs. Above all, in the case of carbon nanotubes which are similar to graphene, since the yield is significantly decreased while carbon nanotubes are subjected to purification after synthesis, the price of a final product is too high even though the product is synthesized by using cheap materials, whereas the unit cost of preparation in the preparation of graphene using graphite as a raw material is low due to high yield after purification. In addition, in the case of single-layered carbon nanotubes, the metallic and semiconductor properties vary depending on chirality and diameter thereof, and all the band-gaps thereof are also different from each another even though the semiconductor properties are the same. Accordingly, a separation process is required for single-layered carbon nanotubes to be applied to a particular application field, but it is known that it is very difficult to perform the process. On the other hand, in the case of graphene, since the electrical properties thereof vary according to the crystal direction of graphene, a user may provide an electrical property in a direction selected by the user, and thus may usefully design a device. Such properties of graphene may be very effectively used in a carbon-based electric or electromagnetic device, and the like.
Currently, examples of a method for preparing graphene include mechanical, epitaxial, thermal expansion, gas phase, chemical vapor deposition (CVD), graphene oxidation-reduction, graphite intercalation compound (GIC) methods, and the like as follows, and the CVD method and the graphene oxidation-reduction method are generally used in the preparation of graphene.
1. Direct Exfoliation Method from Graphite
1) Mechanical Exfoliation
A mechanical exfoliation is used, in which each layer is peeled off from graphite using an adhesive tape, and the like. Although high-quality graphene required for basic studies may be obtained, it is difficult to commercialize the method because the yield is very low, and furthermore, it is difficult to obtain a graphene sheet with a large area (Science, 2004, 306, 666).
2) Exfoliation Using Solvent
This is a method of peeling off a graphene sheet from graphite by using an appropriate solvent without a particular intercalant or a manipulation such as oxidation-reduction. When a solvent system having a solvent-graphene interfacial interaction energy higher than the graphene-graphene interfacial interaction energy is used, a graphene-dispersion solution may be obtained (Nat. Nanotechnol. 2008, 3, 563).
3) Thermal Expansion/Intercalation
Expandable graphite may be heated to 1,000° C. to make a graphite flake composed of a plurality of layers, molecules such as oleum (f-sulfuric acid/SO3) and tetrabutyl ammonium hydroxide may be sandwiched between graphene layers, and the resulting flake may be put into a solution including 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-5000] (DSPE-mPEG) and ultrasonically treated to prepare a graphene-dispersion solution (Nat. Nanotechnol. 2008, 3, 538).
4) Oxidation/Reduction of Graphite
Graphene is oxidized to prepare graphite oxide, and the graphite oxide is exfoliated, and then is reduced to prepare graphene. Double bonds in the graphite oxide compete with oxygen and the graphite oxide usually becomes an epitaxial form, and edges thereof are allowed to have various functional groups including hydroxyl, carboxyl, and other carbonyl groups. The graphite oxide is easily exfoliated through an ultrasonic treatment, and reduced by a reducing agent such as hydrazine to obtain graphene. This is a method of obtaining graphene by oxidizing graphite to become dispersed, and then reducing the graphite, and the method enables graphite to be relatively mass-produced, and thus has been currently used in many cases. However, there is a disadvantage in that oxygen atoms are not completely removed during the reduction process.
2. Epitaxial Growth
This is a method of deposition/growth on various metals or metal-carbides (Si—C, Ru—C, Ni—C, and the like), and the like. The Si—C crystal thermal decomposition method is based on a principle in which when Si—C single crystals are heated, SiC on the surface is decomposed to remove Si, and a graphene layer is produced by the remaining carbon atoms. At this time, methane and hydrogen gas and the like are used at high temperature. This is a method which is not suitable for mass production because high vacuum and high temperature are required when graphene is prepared, and the unit cost of preparation is high.
3. Organic Synthesis Method
This is a growth method through synthesis from low-molecular weight organic compounds. In the aryl-aryl carbon atom bonding reactions hitherto developed, a catalyst such as palladium is used (J. Am. Chem. Soc., 2008, 130, 4216). The preparation of graphene-type compounds by a preparation method using the organic synthesis is still globally in its early stages, but there is a limitation in synthesizing graphene with a large capacity and a large area because the polymerization reaction conditions are demanding and the polymerization also has a very low success rate.
4. CVD Method
The CVD method is a method of obtaining graphene formed on a thin metal film prepared by depositing a catalyst metal on a substrate, flowing a gas including carbon along with argon and hydrogen thereon at a high temperature of 800° C. or more, and cooling the film. However, there is a disadvantage in that the process temperature is very high, graphene may be damaged during the process of removing the catalyst, and the method is adverse in terms of large area and price.
5. Method Using Graphite Intercalation Compound
The method using a graphite intercalation compound is to intercalate a metal into the interlayer region of graphite. The original interlayer interval of graphite is 3.35 Å, but when alkali metal or alkaline earth metal ions are intercalated into the interlayer region of graphite, the interlayer interval thereof is increased. At this time, as ions located in the lower part of the periodic table, that is, ions with a larger atomic radius are intercalated, the interval thereof is further increased. Since the alkali metals and alkaline earth metals are elements corresponding to Groups I and II of the periodic table and are very reactive, it is impossible to perform the process under the oxygen atmosphere. In addition, there is a disadvantage in that the unit price of graphene is significantly increased since the price of the metal itself is also very high.
Therefore, there is a continuous need for a simple and safe method for obtaining graphene with a large area at a high yield, which is the most strongly highlighted material for a carbon-based electric device or a carbon-based electromagnetic device while overcoming the shortcomings of the conventional method.